Avalanche pulse generator- part 2

I build a more permanent version of the pulse generator with some changes. The comparator wasn’t working very well with the 1V reference I was using. I made some measurements with different voltage references and got this:

Vref Vfeedback Vout(measured) (R2/R1+1) Vout(calculated) Error[%]
0,282 1,1 76,8 69,03 19,47 -294,5
0,759 1,087 76,6 69,03 52,39 -46,2
0,953 1,092 77 69,03 65,78 -17,0
1,218 1,258 88,7 69,03 84,08 -5,5
1,243 1,283 90,3 69,03 85,80 -5,2
1,499 1,505 106,3 69,03 103,47 -2,7

The feedback loop was working and the converter regulating but the output value didn’t correspond to the one set it with the feedbacks resistors. The difference between the reference voltage (Vref) and feedback voltage (Vfeedback) is lower with a reference above 1,2V (another source of error is the lack of precision resistors in the feedback divider). and increasing the reference value gives a lower error (difference between the measured and the calculated Vout).  At the end I just put a potentiometer to regulate the reference and tweaked until the converter output was near 90[V].

I started with the oscillator

Multivibrator

Then added the step-up

step up added

And test it

testing the step up

Added the voltage doubler

voltage doubler added

… and test it again…

voltage doubler test

I made a mistake while connecting the oscillator to the step-up. I soldered a cable to the wrong output and was using the inverted signal with a short ON time and a longer OFF time. After changing the output the voltage increased a bit, but  not that much (something like 115[V]). Then soldered in the reference and the comparator.

comparator

Then I connected all together, put the resistor divider to get the voltage feedback and set it to 80V

dc-dc converter set it to 80[V]

Finally soldered the transistor, 50 ohm termination and BNC

Avalanche pulse generator

The board with the different blocks labeled,

Avalanche pulse generator

and finally made some measurements.

Avalanche pulse generator

The formula to calculate the bandwidth based on the rise time is BW=0.35 / rise time [ns], I measured somewhere near 2.5 [ns], but that is a bandwidth of 140 MHz. That’s a lot higher from what I was expecting (I have a DS1052E without the bandwidth hack, so it’s 50Mhz). I’m probably doing something wrong but I don’t know what…

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Avalanche pulse generator- part 1

I wanted to make a fast pulse generator like the one designed by the great Jim Williams1. Unfortunately, the required chip for the high voltage power supply (LT1073) is not available where I live. I found several similar circuits using different approaches to generate the required voltage. I particularly liked the one in Dangerous Prototypes2, in which an astable multivibrator made with transistors is used to drive a step-up and It got me thinking ¿could be possible to “make” the entire LT1073 with transistors?

I looked for the datasheet3 and found a block diagram of the device.

bloque

Figure 1. LT1073 internal block diagram.

Comparator A1 compares the feedback pin voltage (Fb) with the internal voltage reference. When the feedback drops below 212mV the comparator A1 switch on the oscillator. The driver amplifier boosts the signal level to drive the output NPN power switch Q1. The switch cycling action raises the output voltage and the feedback pin voltage. When the feedback voltage is high enough the comparator turn off the oscillator.

Oscillator

I made the oscillator with an astable multivibrator (like the one in DP). I opted for the version with waveform correction4.

osc

Figure 2. Astable multivibrator with diodes for edge correction and a transistor to control it.

I added a transistor to control the oscillator. When the feedback voltage is lower than the reference the comparator goes low and the oscillator is activated in order to rise the output voltage. According to the datasheet the oscillator is set internally for 38µs ON time and 15µs OFF time. I calculated the resistors needed using a 1nF capacitor but later on I tweaked the values in the breadboard until I got close enough to the required ON and OFF time.

NewFile43

Figure 3. Oscillator output.

Voltage reference

For the reference I made a variable zener with a pair of transistors5.

zener

Figure 4. Variable zener.

I set it to 1V and made some measurements to see how stable the reference was:

Figure 5. Reference voltage (yellow) with power supply variations (blue)

The measurements were done with the converter working making the reference voltage quite noisy. Without a 1uf capacitor between Vref and ground (not shown in the schematic) it looked even worse:

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Figure 6. Voltage reference without the 1uf capacitor.

These are the measured values in a much compact graph:

Ref output_vs_vcc

Figure 7. Voltage reference output with variations in voltage supply

Comparator

I build the simplest comparator I could find:

comp

Figure 8. Comparator schematic.

For testing I connected the (-) input to a potentiometer and the other input to the oscillator output:

comp_test

Figure 9. Comparator test setup.

Moving the potentiometer up and down changes the comparator output width:

image

Figure 10. The comparator output is the yellow trace, the potentiometer value is at the top left corner and the blue trace is the oscillator output.

It is not the best comparator you could find but it is good enough for this application. I’m not using hysteresis like the comparator inside the Lt1073 does.

Switching transistor

Tried first with a single BC548 but couldn’t get more than 30V, adding a 2N2222 in a Darlington configuration I could reach a little more than 50V:

Sw

Figure 11. Output Darlington switch.

I also added a diode-capacitor voltage step-up network (as in the original circuit with the LT1073) and this is what the final circuit looks like:

sch

Figure 12. Complete schematic.

and the block diagram of my version:

bloque2

Figure 13.

Then did some test to see if the whole thing was regulating properly and how the output changed with voltage supply variations.

HVoutput_vs_VCC

Figure 14. Output (yellow) vs power supply (blue).

Again the values from the previous oscilloscope captures in a single graph:

HV output_vs_vcc

Figure 15. High voltage output from the converter with variations in the power supply. The converter is configured to output a little bit more than 82V.

I also made some captures of the output from the oscillator as the supply voltage decrease:

osc(blue)_HV output(yellow)_variable VCC

Figure 16.

It can be seen how the pulse trains increase with the reduction of voltage supply. With voltages lower than 1,7 V the converter can’t regulate properly.

I was able to source locally the 2N2369 used in the original application. I connected it in the protoboard to see if the converter could get it to avalanche. The blue trace (Figure 16) shows the voltage in the capacitor, it charges until the voltage is high enough to avalanche the transistor rapidly discharging and generating a fast rising pulse shown in the yellow trace.

NewFile39

Figure 17.

All this testing was done on the protoboard so is no surprise that the pulse looks like crap:

NewFile38

Figure 18. Collector capacitor discharge (blue) and pulse from the avalanche transistor (yellow). It can be seen that the breakdown voltage is around 67[V]

I used a 22pF capacitor but even without using one the parasitic capacitance in the protoboard where high enough to make it work anyway. Now I need to build a proper board.

Reference

1. Application Note 72, APPENDIX B, Measuring Probe-Oscilloscope Response
2. Avalanche pulse generator, and some scope porn
3. LT1073 Micropower DC/DC Converter Adjustable and Fixed 5V, 12V
4. Transistors Tutorial, Part 7: “Oscillators”
5. Simple Transistor Circuits For Experimenting, Fun and Education. Variable Zener Diode